By using a photocoupler, signal transmission is possible in a state in which the input terminals are electrically insulated from the output terminals.
Therefore, the photocoupler can be used to transmit signals in an electronic device that includes different power supply systems such as a DC voltage system, an AC power supply system, a telephone line system, etc.
In the case where the signal to be controlled on the secondary side is a direct-current voltage, the photocoupler may have a Form C contact operation such as that of a mechanical relay by combining a normally-off transistor and a normally-on transistor.
In such a case, it is necessary to maintain good balance of the operating currents of the two signal paths while maintaining a small chip size.